The invention relates generally to the use of pressurized air in high speed production line type machines and more particularly to apparatus for varying the pressure of air used in a variable speed metal can inspection machine.
The present invention is particularly adapted for use in the inspection of metal can bodies by a machine such as described in U.S. Pat. No. 4,074,809 issued to McMillin et al., which is hereby incorporated by reference. In machines of this type pressurized air is used to aid in forming a light tight seal at one end of a can body member. Air is also used to pressurize the interior of can body members to aid in the detection of can body defects such as pin holes or small slits in the can body wall and irregularities in the rim surface of the can body.
Machines of the type described in McMillin et al are operated over a relatively broad speed range, typically from approximately 600 to approximately 1200 cans per minute, to accommodate the changing rate of can delivery from an associated infeed track. An even broader dynamic operating range is generally desirable; however, it has been found that the amount of air pressure which must be applied at an air manifold to achieve a desired sealing pressure is not constant with the machine speed. A pressure setting at the manifold sufficient to provide proper seals at a low speed setting is insufficient for sealing at higher speeds. Conversely, higher pressure settings which allow adequate pressurization of can bodies and seals at higher speeds cause the seals to be blown off at lower speeds. The improper seal formation produced under either condition causes an associated photomultiplier to sense the pressence of light and reject the can body associated with the improperly formed seal whether or not is is actually defective. Thus, either a high or low pressure setting will result in increased scrap. A high pressure setting will also result in physical damage to the sealing apparatus at low speed operation. The problem may not be avoided by running the machine only at the high speed setting because of problems associated with high speed loading and the presence of empty pockets within the machine.
Thus it is desirable to provide apparatus to vary the pressure of air used in forming light tight seals and in pressurizing can bodies to allow a can body inspection machine to be operated over a very broad dynamic range. It is also generally desirable to provide apparatus which may be adapted for use in other high speed machines using pressurized air having pressure parameters which vary with the machine operating speed.